Fix test for CannotAuthenticate when metadata is missing

[Copilot]

Fix Build Error in Authentication Test

• Understand the issue: PR Add auth regression test for missing resource in PRM #1265 added test with new Uri() but PR Change ProtectedResourceMetadata URI properties to strings and build resource strings directly #1264 changed AuthorizationServers from List<Uri> to List<string>
• Fix line 545: Change new Uri(OAuthServerUrl) to OAuthServerUrl (string)
• Verify the fix by running build - Build succeeded with no errors
• Run the specific test to ensure it works correctly - Test passes
• Run all AuthTests to ensure no regressions - All 26 tests pass across net8.0, net9.0, net10.0
• Run full test suite to verify entire codebase

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[halter73]

This fixes the build break in main that I caused by merging two PRs without merging the second with main first and retesting.